I know. I have taken the approach that it is for the better in the long run (when did I become my parents?). I hate it with every fiber of my being, but I learn what my child loves, so I can take it away from him when he doesn't do what is right.
Not fair, cool, or what I ever wanted. He is a willful brat though (13) and whipping is out as it never made an impression and he is too old now anyway.
If you do make the ultimatum, you need to track him every day and warn him if he slips. If he fails to meet the goals, you have to yank him.
Again, just my opinion. If you ever need a sympathetic ear, just shoot a PM. I agree that parenting is hard, but there are so many priceless moments as rewards.